Humans tend to organize information in categories. The categories in which information is organized are themselves typically organized relative to each other in some form of hierarchy. For example, an individual animal belongs to a species, the species belongs to a genus, the genus belongs to a family, the family belongs to an order, and the order belongs to a class.
With the advent of computer systems, techniques for storing electronic information have been developed that largely reflected this human desire for hierarchical organization. Conventional computer file systems, for example, are typically implemented using hierarchy-based organization principles. Specifically, a typical file system has directories arranged in a hierarchy, and documents stored in the directories. Ideally, the hierarchical relationships between the directories reflect some intuitive relationship between the meanings that have been assigned to the directories. Similarly, it is ideal for each document to be stored in a directory based on some intuitive relationship between the contents of the document and the meaning assigned to the directory in which the document is stored.
In contrast to hierarchical approaches to organizing electronic information, a relational database stores information in tables comprised of rows and columns. Each row is identified by a unique RowID. Each column represents an attribute or field of a record, and each row represents a particular record. Data is retrieved from the database by submitting queries to a database server that manages the database. The queries must conform to the database language supported by the database server. Structured Query Language (SQL) is an example of a database language supported by many existing database management systems.
Each type of storage system has advantages and limitations. A hierarchically organized storage system is simple, intuitive, and easy to implement, and is a standard model used by most application programs. Unfortunately, the simplicity of the hierarchical organization does not provide the support required for complex data retrieval operations. For example, the contents of every directory may have to be inspected to retrieve all documents created on a particular day that have a particular filename. Since all directories must be searched, the hierarchical organization does nothing to facilitate the retrieval process.
A relational database system is well suited for storing large amounts of information and for accessing data in a very flexible manner. Relative to hierarchically organized systems, data that matches even complex search criteria may be easily and efficiently retrieved from a relational database system. However, the process of formulating and submitting queries to a database server is less intuitive than merely traversing a hierarchy of directories, and is beyond the technical comfort level of many computer users.
In the past, hierarchically organized systems and relationally organized systems have been implemented in different ways that were not compatible. However, some relationally organized systems incorporate features that allow the systems to emulate a hierarchically organized system. This type of emulation is especially desirable when the storage capability and flexibility of a relational system is needed, but the intuitiveness and ubiquity of the hierarchical system is desired.
Relationally organized systems that emulate hierarchically organized systems provide mechanisms that allow access to data as both hierarchically organized data (“hierarchical access mechanisms”) or that allow access to data as relationally organized data (“relational access mechanisms”). It is desirable that access to the items of data be controlled consistently, whether accessed through relational access mechanisms or hierarchical access mechanisms. For example, in a relationally organized system that emulates a hierarchically organized system, a user who is not allowed access to a particular item of data that is the hierarchical parent of child items of data, is also not allowed to access the child items through a hierarchical access mechanism. The parent item and child item may be stored in rows of a relational table, which may be accessed through a relational access mechanism by issuing a query that requests data from the table. Access control that is uniform between the hierarchical access mechanism and the relational access mechanism does not permit using the relational access mechanism to access the rows of the relational table that store data for the children items.
Based on the foregoing, it is clearly desirable to provide a mechanism, usable in relational database systems, that provide uniform access control to relationally organized data and hierarchically organized data.